In the course of conducting space activities involving the growing of crystals under zero gravity conditions, the National Aeronautics and Space Administration first had a need to measure the thermal diffusivity of mercury-cadmium telluride (HgCdTe) melts by the infrared laser pulse method. However, such measurements of materials with high vapor pressure have not been commonplace in the past. In the past, diffusivity measurements worked with materials of low vapor pressure only. Therefore, because of the high vapor pressure of mercury, prior diffusivity measurement techniques were not applicable to mercury-cadmium telluride. To develop such techniques for mercury-cadmium telluride, optical cells capable of withstanding internal pressures of about 100 atmospheres at temperatures up to about 1000.degree. C. were required to perform the necessary tests. In addition, the cells had to be transparent and capable of passing light without distortion, so that the chemical in the cell could be pulsed by a laser, observed under a microscope or that high quality photographs could be made. However, it was found that optical cells capable of passing light without distortion and also capable of handling such high temperatures and pressures are not known in the prior art and therefore are not available on the commercial market.
What is state-of-the-art and also available on the commercial market is a type of optical cell having windows made as thin lids fused over the ends of the cylindrical center portion. The windows on this type of cell are deliberately made thin so as to avoid distortion when they are fused to the body. Therefore, this type cell does not have the structural strength needed to handle high pressures.
Therefore, the object of this invention is to provide an optical cell with flat faces to hold mercury-cadmium telluride melts for the measurement of the thermal diffusivity by the infrared laser pulse technique. Another object is that the optical cell have the following features: (1) must be infrared transparent, (2) must withstand internal pressures of the order of 100 atmospheres at temperatures up to 1000.degree. C., (3) must not react chemically with mercury, cadmium, tellurium, or the probable impurities of these elements, (4) must be hermetically sealed, and (5) must not have voids or vapor spaces in the optical cavity when filled.